Recently, since stainless steel is applied to an outer case of important home appliances, premium products having the feel of metal and sturdiness are becoming a main trend. However, an outer case formed of a stainless steel becomes unclean easily because of fingerprints, for example. To prevent this, coating for protection is required.
A typically applied coating layer is formed of a polymer (e.g., acrylic or urethane) that is applied on the surface of a stainless steel. Urethane polymer can be obtained by the reaction of a polyol-based compound and isocyanate. An adequate catalyst and hardener are commonly added during the reaction. Through the reaction of a polyol-based compound and isocyanate, the urethane polymer is synthesized and, because its reaction of amine and isocyanate is fast enough, no catalyst is not required.
The urethane polymer has an excellent abrasion-resistance such that it contributes to an additional chemical combination of a carbonyl group (c═o) in a urethane chemical combination. Moreover, the urethane polymer has compression strength because of strong heat-resistance and outstanding three-dimensional stability. The urethane polymer also has characteristics such as elasticity, acid-resistance, alkali-resistance, chemical-resistance, and ultraviolet ray-resistance. Furthermore, urethane polymer does not pollute environment and is not affected by a temperature. Thus, the urethane polymer is not limited by temperatures even in winter.
A urethane resin is applied on a steel sheet or an aluminum sheet in order to protect its surface from scratches or is used for providing chemical resistance. If solid contents of the urethane resin is less than 10% weight, salt water resistance for penetration of corrosive ions and chemical resistance for penetration of chemical materials are not sufficient such that chemical resistance and alkaline resistance are deteriorated. Furthermore, in order to enhance high temperature and humidity resistance and freezing resistance, an acrylic resin is added or is copolymerized with the urethane resin.
Each above-mentioned component is melted in a solvent in order to form a coating liquid, and then is applied on the surface of stainless steel. Then, a drying process is performed on the result. The drying process is performed to remove the solvent contained during the coating process. After the drying process, a coating layer is hardened. The hardening process removes a remaining solvent of the drying process, and also hardens the coating layer.
Urethane polymer used as a coating layer material of stainless steel according to a related art is used as a coating material because of excellent characteristics such as water-resistance, chemical-resistance, and glossiness, but because polyamine constituting the urethane polymer is expensive, its applications are limited. Moreover, because the coating layer formed of only polymer has a pencil hardness of 1 H, scratches and cracks occur easily, and also because of fingerprints on the outer cases of home appliances, formed of stainless steel, it is difficult to maintain cleanliness.
Furthermore, a UV hardening method is used in a coating process. However, pigments for the coating process are expensive compared to related art pigments and also new UV hardening equipment needs to be purchased. Moreover, a hardening process may not be appropriately applied to an edge or uneven portion (i.e., not a flat portion). Additionally, a heat of a high temperature, generated in a UV lamp, may deform the external appearances of products. Furthermore, it is difficult to prevent heat deformation, and its cost is expensive.